The present invention relates to a system for installing a heater unit assembly which is used for effecting annealing treatments, particularly for annealing embrittled reactor vessels.
During the normal operation of a nuclear reactor, the reactor vessel, which is normally made of steel and which houses a core containing nuclear fuel, is exposed to intense radiation. Experience has shown that this radiation causes changes in the fine grain structure of the steel walls of the vessel. These structural changes make the walls brittle, a problem commonly referred to as reactor vessel embrittlement. Embrittlement reduces the flexibility of the vessel wall and increases the susceptibility of the vessel wall to fracturing, particularly if subjected to sudden stresses, such as due to operating transient events and pressurized thermal shock events.
Because of this embrittlement phenomenon, the United States Nuclear Regulatory Commission requires that a reactor vessel be removed from service when embrittlement reaches a predetermined stage, thus ending the useful life of this portion of the nuclear power plant. Replacement of such a vessel is extremely expensive because the vessel is built into and is a part of the reactor containment building, thereby making replacement economically impractical.
In order to deal with this problem, it has been proposed to subject such a vessel to annealing in place in order to restore the ductility and toughness of the metal constituting the reactor vessel and to perform this annealing with a heater unit assembly installed in the reactor vessel.
In order for such an annealing process to be performed successfully, it is necessary that the heating of the vessel wall be heated in a uniform manner and this requires that the heater unit assembly be accurately positioned in the vessel, and in particular that it have an accurately defined angular orientation relative to the vessel axis.
Since the construction of reactor vessels which are now in use did not take account of the need for such a subsequent annealing treatment, these vessels were not provided with any structures intended to permit proper alignment of annealing equipment. However, most such vessels are provided, on their interior surface, with guide members for aligning structures associated with the reactor core.